Fixes for a number of tests, cmake files and context applet metadata.

Fix a number of tests.

Add KLocalizedString::setApplicationDomain to various tests to avoid extensive kf.i18n warnings in test outputs.

Add default website to context applet metadata to pass AppStream requirements.

Add extra SercviceType "KPackage/Generic" to context applets to make kpackagetool5 happier with them.

Update some deprecated cmake commands.

Change contextapplet.desktop servicetype from Plugin to ContextApplet; the Plugin seemed to originate from the amarok-plugin.desktop file, from which the file was initially copied from.

These changes shouldn't affect any actual functionality, but as cmake files and the context applet metadata are altered, there's breakage potential, so I'll be testing installation in a clean system test too see everything still works.